Abradable seals are used in turbomachinery to maintain very close clearances between spinning blades and the surrounding case structure. They are comprised of materials which are particularly adapted to fragment and disappear when contacted by rotating parts which are spinning at high speeds. As generally mentioned in U.S. Pat. Nos. 3,879,831 to Rigney et al, 3,084,064 to Cowden et al and 4,257,735 to Bradley et al, abradable seals must have a particular combination of properties. They must be resistant to erosion from high velocity gas streams which at times carry with them fine particulates while at the same time being capable of disintegrating when contacted by the tip of a high speed blade or by a knife edge of a labyrinth rotating shaft seal, so such parts are not substantially degraded. Such a mode of behavior is highly desirable because at times a rotating part of an engine and surrounding casing having the abradable seal may come together too closely. When this occurs locally around the circumference, it is highly desirable that the casing having the abradable seal sustain all the wear so that the clearance between the rotating part and the case elsewhere around its circumference will not be increased and leakage will not be unduly increased.
Abradable seals in the compressor part of gas turbine engines are generally used at 540.degree. C. or less and thus, they are usually comprised essentially of metal. For example, U.S. Pat. No. 3,350,178 to Miller describes how a porous metal structure is made by combining a fugitive polymer with metal particles and suitably sintering the mass. Commercial products of sintered powder metals may in fact be presently used in certain engines. Another type of seal material is a mat of metal fibers having a felt-like construction. This type of material is described in the aforementioned Bradley et al patent and U.S. Pats. No. 3,701,536 to Matthews et al, 4,139,376 to Erickson and 4,273,824 to McComas et al. To some, the fiber metal configuration is preferred because of the more continuous nature of the wrought wire structure, compared to a powder metal product. As indicated in the patents referred to, such fiber metal structures may have densities in the range of 10-50 percent, more typically 14-35 percent; but for use with knife edges they have densities in the range 14-23 percent. Thus, fiber metal seals tend to be less dense than the sprayed powder metal seals which are the subject of the present invention. In fact, sprayed powder metal seals cannot feasibly be made with so little density and still function effectively.
As just suggested, abradable seal material has also been made by plasma arc spraying, and it is with such kinds of seals that the present invention is particularly concerned. Plasma arc spraying of abradable materials is described in the aforementioned Cowden et al patent, the disclosure of which is hereby incorporated by reference. Different users have their varying preferences among the foregoing kinds of abradable seal materials. Generally, there is a preference for the seal materials made out of powder metals because powder metals tend to be more readily available in diverse compositions and tend to be lower in cost than fiber metals. There are technical differences in performance between the different kinds of seals and it was to overcome a deficiency of the seals made from sprayed powder metal, that the present invention came about.
Pat. application Ser. No. 565,542 filed on Dec. 27, 1983 by the present applicant and others, now U.S. Pat. No. 4,759,957 describes in detail the way of making a porous metal abradable material by plasma arc spraying nichrome powder and polymethyl methacrylate powders. The seal material referred to in the application is an improvement on the general kind of material which can be made by plasma arc spraying in which a metal powder and a fugitive polymer powder are sprayed onto a substrate. Heating is then used to drive away the polymer; when 80 Ni 20 Cr metal is sprayed the resultant structure has a density of about 26-40 percent of the parent metal.
Sprayed seals of the foregoing type are cost effective and can be quite useful, although they continue to undergo development and refinement. While they generally do perform the function that is set forth above, it has been observed that there is sometimes a propensity in knife edge labyrinth seals for the seal material to prematurely disappear from the place where it has been deposited. Such an effect would not be surprising if it were confined to the region where the seal material tends to be contacted by the rotating knife edges, as illustrated in the Bradley and Matthews et al patents. However, disappearance of the material has been noted in regions where there is no possibility of contact or interaction with the rotating parts, specifically, in the portion which is between two spaced apart circumferential outside diameter knife edges, e.g., region 27 in FIG. 3 of the Matthews patent, being the same as 36 in FIG. 1 herein. And when the material between the knives disappears, this apparently makes the remaining material more prone to being undercut and to being unacceptably disintegrated. The clearances between the rotating part and the circumbscribing static structure become greatly excessive and there is undue leakage and resultant loss in efficiency of the turbomachine.
Research was undertaken to identify the cause of the observed limitation. The failure mode was associated with the plasma arc sprayed seal and was not observed in a similarly placed substitutional fiber metal seal. Further, the failure was only observed when the plasma arc sprayed deposit was confined between the two knife edges of a labyrinth type seal and it was not particularly observed in a more prevalent application where the abradable material is contacted by rotating blades. While identifying the mode of failure was difficult owing to the conditions under which it occurs, the opinion has been formed that the cause is "self-erosion" (also called "auto-erosion"). It appears that particulate matter which comes loose from the abradable seal material in the ordinary course of operation will not flow downstream through the machine as one might expect. Instead, this particulate is trapped and is thrown outwardly by centrifugal force between the knife edges. As it rotates circumferentially around the interior of the circumscribing structure, the quantity and impact of the debris is evidently sufficient to degrade the seal material surface. More particles are dislodged and in a very short time there is a "chain reaction" which causes a substantial amount of the abradable seal material to disappear. The phenomenon is peculiar to circumstances where the particulate are trapped between knive edges. The effect is not observed in situations where there is significant leakage across the sealed region, such as where the knife edge seal is eccentric with the circumscribing abradable material, or when spaced apart blade tips contact the material. This supports the hypothesis since it is supposed that under such conditions the abraded away and other foreign particles known to be present are able to flow away downstream.
Consequently, there has been a search for ways to improve seal materials to make them resistant to self erosion. However, as mentioned above there is a paradox in the materials engineering design of seals in that if they are made stronger in order to become more resistant to particulate erosion, then they also tend to become less functional in that they will not be as abradable and instead will unacceptably wear away the knife edges.